Optical brightener for aqueous high yield pulp and process for making brightened paper

ABSTRACT

Paper is brightened by addition of a tetrakis(hydroxyalkyl) phosphonium (“THP”) salt in an amount greater than 0.04% but no more than 5% by weight in wet pulp. The amount of THP salt added is sufficient to enhance the brightness of paper made from fibers in the wood pulp to a desired brightness level. The THP salt is commercially available and previously used as a fire retardant and a biocide. Though particularly useful in the production of brightened cellulose fibers of high yield pulp, with the benefit of generally improving opacity, the process may be used to treat all wet pulp which is to be brightened.

This application claims the benefit of the filing date of Provisional Application 60/542,381 filed Feb. 6, 2004.

FIELD OF THE INVENTION

This invention relates to brightening paper made from high yield, or ultra-high yield wood pulp which may be bleached with a bleaching agent, either before it is brightened, or concurrently with being brightened with a brightening agent. Wood or paper pulp which yields greater than 60% preferably 80% cellulose fibers upon recovery, having removed the bulk of its non-cellulosic components, is referred to as “high-yield” pulp. A pulp which contains an even higher percent of cellulose, such as may be obtained when processing recycled paper, is referred to as “ultra-high-yield” pulp. Depending upon the source of recycle paper, it may have been made from either high-yield or ultra-high-yield, each of which, separately or in combination, may be fed as wet pulp to the feed tank of a paper machine and then to the belt of a paper machine. The term “recycle paper” refers to paper which is obtained from a paper recycling facility, and may include waste paper which was discarded before it was used for any specific purpose. The term “wet pulp” as used herein refers specifically to an aqueous dispersion of cellulose fibers, fed to the belt of a paper machine, in a high enough concentration to be used for making paper in a paper machine, typically in the range from 0.1% to 15% solids, “solids” being the residue on a Buchner funnel fitted with a #4 Whatman filter paper, which residue of mainly cellulose fibers is dried.

The Problem

Irrespective of the manner in which it is made, wood pulp to be used in making paper is desirably brightened, even if the finished paper is going to be off-white or distinctly colored, and not high quality. Such brightening is typically done with an optical brightener added to the pulp. However, because at present, even starting with a high quality wet pulp of recycle paper, a relatively high amount of an optical brightener must be added to achieve a brightness of about 75 (measured as described below), the cost of such brightening is relatively high, thus making it difficult to justify such brightening for recycled paper and virgin paper of comparable quality.

BACKGROUND OF THE INVENTION

Brightness is a hallmark of high quality paper; it is a key specification for paper irrespective of its use, and paper makers are continually pressured to improve the brightness of papers they produce. Imparting a high degree of brightness to recycled paper or “waste paper” made from pulp which is typically colored, is a challenge. The extent to which recycled pulp (that is, pulp derived from recycled or waste paper) is colored is a function of the source of paper from which the recycled pulp is derived, and the extent to which the pulp may have already been bleached before it is fed to a paper machine.

Virgin paper on the other hand, is derived from virgin pulp which is typically less colored than pulp of recycled paper, irrespective of how the virgin pulp is produced. It may be made from high yield wood pulps derived from “groundwood” and “refiner mechanical” pulps or by “thermomechanical pulping”. Groundwood pulp is produced by abrading a log against an abrasive rotating grindstone. Pressurized groundwood is made by abrading logs in a pressurized atmosphere to allow use of a high temperature which generates steam held under high pressure in the range from 5 to 20 atmospheres. Such pulp is typically relatively dark in color and, if it is to be used for making high quality paper, though bleaching alone improves brightness, the pulp is generally bleached before it is treated with an optical brightener.

Refiner mechanical pulps are produced by feeding wood chips into the open eye of a “refiner” comprising especially designed grooved discs rotating at high speed; as the chips travel from the eye of the refiner to its periphery, the chips are progressively comminuted into ever smaller wood fragments until they become a mass of substantially separate fibers. In thermomechanical pulping wood chips are steamed under pressure for a short time prior to being fed to a refiner.

High-yield wood pulps, obtained from any one of the above processes, are commonly used in newsprint, printing papers, molded products, corrugated paper, boxboards, and other applications, but only select applications among the foregoing justify the cost of bleaching and/or brightening.

It must be borne in mind that a single percentage point in brightness, is notable, and 10 percentage points is deemed a high gain in brightness. Such a high gain may be observed when using very poor quality wet pulp to make paper. A relatively high level of brightening is obtained simply by oxidative bleaching, for example, with hydrogen peroxide which can produce a brightness gain of 5-15 percentage points depending upon the severity of treatment, that is, concentration of hydrogen peroxide used, and, the time of treatment of the pulp. However, such oxidative bleaching is expensive. Reductive bleaching is cheaper but the gains made in brightness are relatively small. Such bleaching is accomplished with compounds such as sodium hydrosulfite, also referred to as sodium dithionite or formamidine sulfinic acid (FAS), optionally in the presence of additives such as sodium tripolyphosphate, trisodiumnitrilotriacetate, and tetrasodium ethylenediaminetetraacetate. Not surprisingly, such bleaching also results in a substantial improvement in brightness, as a result of which sodium hydrosulfite is often also referred to as a brightener.

Irrespective of how paper is brightened, there is no known theoretical basis for predicting the efficacy of a “brightener”, or even if a compound will function as a brightener at all, much less the conditions under which it is likely to be effective.

Tertiary or quaternary ammonium compounds have been used to remove dyes from recycled fiber (see U.S. Pat. No. 4,710,267 to Elsby et al) but these ammonium compounds having a carbonyloxy portion are unrelated to phosphonium salts.

One must first make handsheets before one can measure brightness. Handsheets to measure the reflectance of pulp, are made as described in Technical Association of the Paper and Pulp Industry (TAPPI) Standard Method T272 sp-97, details of which are incorporated by reference thereto as if fully set forth herein. The level or degree of diffuse brightness is then measured in accordance with TAPPI Standard Method T525 om-86. Since the optical properties may change significantly during the first few hours after a handsheet is made, for control purposes, all measurements are made after allowing each handsheet to stabilize in air at 21° C. (70° F.) for 4 hours.

As will be evident from Table I (herebelow) a specific feed of recycle pulp, obtained from recycling high quality magazines and deinked, by itself and without benefit of any added bleach or brightener, has a brightness value of 69.8 which is passable for low quality white paper. Adding brightener to improve the brightness by about a single percentage point to 71.5 is highly desirable, because it advances the paper made from the same pulp to a higher level of quality. The “ISO Brightness” is a method of measuring brightness developed by the International Organization for Standardization and is well known to those in the art.

Phosphonium salts used herein as brighteners are known compounds which have been widely used as fire retardants. More recently they have been used as biocides in water treatment in cooling towers, chemical process water and in oil fields, but such use has been limited to small concentrations no greater than about 150 ppm (parts per million). There is nothing to suggest such salts should be effective optical brighteners.

SUMMARY OF THE INVENTION

A process is disclosed for enhancing the brightness of a an aqueous dispersion of wood pulp comprising contacting the pulp with a tetrakis(hydroxyalkyl) phosphonium (“THP”) salt in an amount greater than 0.04% in wet pulp, sufficient to enhance the brightness of paper made from the pulp to a desired brightness level, but no more than 5%, the phosphonium salt being represented by the structural formula:

wherein R represents alkyl having from 1 to 4 carbon atoms, preferably CH₂, or C₂H₄; and, X represents a suitable and desirable anion selected from the group consisting of HCO₃ ⁻¹, CO₃ ⁻², PO₄ ⁻³, HPO₄ ⁻¹, HPO₄ ⁻², OH⁻¹, Cl⁻¹, Br⁻¹, HSO₃ ⁻¹, SO₃ ⁻², HSO₄ ⁻¹, SO₄ ⁻², CH₃CO₂ ⁻¹ and CH₃SO₄ ⁻¹, most preferably SO₄ ⁻².

In particular, recycle paper derived from an aqueous suspension of cellulose fibers forming “wet pulp” which is deinked, is brightened by a process comprising, treating the wet pulp with at least enough THP salt to result in a concentration of from 0.05% to 5% THP by weight in the wet pulp fed to a belt of a paper machine, preferably 0.5%; maintaining the pulp in contact with the THP for from about 10 to 90 minutes, preferably at elevated temperature in the range from about 30° C. to 90° C., to provide sufficient time for the fibers in the pulp to sorb the THP, forming THP-coated fibers; feeding wet pulp containing from about 0.1 to 15% by weight of the THP-coated fibers, to a paper machine, to form a web of wet pulp; removing water from the wet pulp; drying the resulting web of paper; and measuring a brightness of the web of paper which brightness is at least 0.5 point higher than a brightness measured for the same pulp without being contacted with the THP, the brightness being achieved with no decrease in opacity relative to that of paper made with the same wood pulp but untreated with the THP or THP salt.

At levels higher than 5% by wt of wet pulp the THP unexpectedly causes lumping of fibers and produces no substantial enhancement of brightening compared to a level in the range from about 2.5 to 3.5% THP; moreover, with an excess of THP, higher than 5%, the strength of the brightened paper may be compromised.

A solution of the THP salt is effective without the presence of a polymer to help coat the THP on fibers. However, the effectiveness of THP is undiminished by the presence of a polymer such as a binder, should a binder be desirable. Moreover, the strength of paper made from THP-treated pulp, treated in the range stated, is substantially the same as the strength of paper made from the same pulp, fed to the same paper machine at the same solids content, and having the same thickness. The strength data was obtained by first forming handsheets from pulp according to TAPPI Standard Method T205 om-88, then measuring the strength of sheets formed according to TAPPI Standard Method T494 om-88.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Typically, attempts at increasing the brightness have included the use of multiple-stage bleaching with hydrogen peroxide or chlorine-containing oxidative bleaches which adversely affect the pulp strength. In addition, the chlorine containing bleaches may contribute to the formation of chlorinated organics which are environmentally undesirable. Methods to recover the loss of strength have included adding expensive kraft fiber or bonding agents to the pulped material. (see U.S. Pat. No. 5,5600,805 col 1, lines 21-29).

THP is only available commercially as an aqueous solution. Despite its having been in commercial use for many years, all attempts to prepare THP as a stable solid, e.g. by recrystallising from glacial acetic acid and vacuum drying, have proved unsuccessful due to the highly hygroscopic and unstable nature of the solid product. To date, lack of a stable solid form of THP has severely curtailed any desire to use it in a commercial, large scale operation such as in treating wet pulp.

In the following detailed description, the particular THP used is tetrakis (hydroxymethyl) phosphonium sulfate (“THPS”). The water content of the pulp to be treated with THPS is such that the pulp is stirrable and preferably, readily pourable on to the belt of a papermachine. A concentrated wet pulp for use in a paper machine contains from about 8 to 12% solids; a wet pulp of normal concentration contains from 3 to 6% solids. The desired concentration of aqueous sodium hydrosulfite or THPS (though the former also functions as a bleach, each is referred to as a “brightener” herein) is metered into a stirred feed tank for a paper machine, and heated for 20 min preferably in the range from 35° C. to 75° C. The pH of the wet pulp is preferably from neutral (6-7) to slightly alkaline (about 9) though the pH may be either substantially higher or lower for short periods of time. It is a requirement of many papermills that whatever the brightener used, it shall not be adversely affected if used in a wet pulp which, for a short period of time less than 1 hour, may have a pH as low as 4 and as high as 12. Of course, a pH which is either too high or too low is immediately adjusted; typically the pH tends to be too low and it is adjusted by the addition of a known base such as an alkali metal hydroxide or preferably with ammonia.

A sample of treated and heated wet pulp is then removed and poured into the sheet machine to make a handsheet.

Unbleached recycle pulp is made from deinked magazine stock by adding 5 lb (2.27 kg) of shredded and digested solids to 20 liters of water at room temperature 25° C. (65° F.) in a Laboratory Pulper (Adirondack Machine Company, Glynns Falls, N.Y.). Repulping was carried out over a period of 15 minutes. The resulting pulp slurry (containing 4.2 percent solids) was used as the supply of pulp for the following examples.

For each example, several runs are made with varying concentrations of brightener in the wet pulp. The rate at which wet pulp is fed is such that the dried handsheet formed has a thickness of approximately 0.09 mm (0.0035″). The consistency (% dry solids) of the wet pulp used for the examples varies from about 0.4% to 9% and the concentration of brightener is varied as indicated in the Tables.

In the Table 1 below, the wet pulp of deinked magazine stock is fed at 2% consistency after being heated to a temperature of about 49° C. (120° F.) while being agitated for 20 min; the concentration of brightener in the wet pulp fed, is given for each run. TABLE 1 Run No. % sodium hydrosulfite % THPS Brightness 1 0 0 69.8 2 0.25 0 70.7 3 0.50 0 70.7 4 0.75 0 70.8 5 1.0 0 71.5 6 0 0.25 71.5 7 0 0.5 72.3 8 0 0.75 72.7 9 0 1.0 73.2

As evident from the above, an unbleached and unbrightened handhseet of recycle magazine stock is relatively bright in relation to typical bleached virgin stock for white paper. Concentration of 0.25% sodium hydrosulfite provides a 0.9 boost to a 70.4 value indicating a relatively high improved over the control, unbleached handsheet; doubling the concentration provides no additional boost; and further increasing by 0.25% to 1% provides only a further 0.1 boost.

On the other hand, the same 0.25% concentration of THPS immediately results in a boost to 71.5 which is higher than that which could be obtained with sodium hydrosulfite except at four times that concentration. Doubling the concentration to 0.5% THPS provides a boost to 72.3 which is 0.8 above the level at 0.25%; tripling the concentration to 0.75% provides a boost to 72.7 which is 0.4 above the level at 0.50%; and, at four times the 0.25% concentration provides a boost to 73.2 which is 0.5 above the level at 0.75%. Thus, it is evident that at four times the 0.25% level, THPS provides a much greater boost than one might expect if one was to accept the comparable performance of THPS as applied to that of sodium hydrosulfite.

In the Table 2 below, another wet pulp of a different less bright deinked magazine stock (from the one tested in Table 1 above) is fed at a relatively high concentration of 8.51 % consistency after being heated to a temperature of about 60° C. (140° F.) and agitated for 20 min; the concentration of brightener in the wet pulp fed, is given for each run. Again the amount of wet pulp poured into the sheet machine is sufficient to make a dry handsheet 0.09 mm (0.0035″) thick. TABLE 2 Run No. % sodium hydrosulfite % THPS Brightness 1 0 0 66.4 2 0.5 0 67.0 3 1.0 0 66.5 4 1.5 0 65.1 5 2.0 0 66.0 6 0 0.5 67.2 7 0 1.0 67.9 8 0 1.5 68.1

From the above it is evident that, as in Table 1, a large boost of 0.6 is obtained with a 0.5% conc of sodium hydrosulfite alone, but a larger boost of 0.8 is obtained with THPS alone at the same 0.5% conc. Further, there is no meaningful increase in brightness with successive increases of sodium hydrosulfite; in contrast, there is an initial bigger boost (of 0.8) with 0.5% THPS and each subsequent increase in THPS results in an appreciable boost from the previous level. As one might expect, the boost from 1.0% to 1.5% THPS is rapidly declining relative to the boost obtained from 0.5% to 1%.

From the above data it is evident that THPS provides an unexpected and highly beneficial boost in brightness in a restricted range of from about 0.05% to 3% THPS conc in a wet pulp, and there is little point in increasing the concentration up to 2% where the risk of this particular fiber lumping and diminished strength become unnecessarily high.

In each of the foregoing examples sheets of paper made from fibers deposited from untreated wet pulp, without conventionally bleaching the fibers with hydrogen peroxide, chlorine-containing oxidative bleaches, or any other conventionally used bleaching agent, each sheet had an opacity in the range from 75 to 80. After the same wet pulp was treated as described in this process with the four concentrations from 0.25% to 1% by wt of THPS in the wet pulp, the opacity of the fibers was increased from 1 to 10%, the higher the concentration of THPS, the higher the percentage increase in opacity.

In the examples above, only the sulfate of the phosphonium compound was used. It has been found that the phosphonium cation and the anion each have the ability to bind different impurities in the wet pulp, thus brightening the cellulose fibers by purifying their immediate environment rather than conventionally bleaching the fibers with hydrogen peroxide or chlorine-containing oxidative bleaches. In particular, the sulfate SO₄ ⁻² anion effectively binds to Fe⁺ cations in the aqueous phase, removing them from contact with the fibers. The phosphonium cation binds to ioninc coloring agents including certain organic impurities. Thus, different phosphonium anions are expected to bind to certain different cations preferentially, and differently substituted phosphonium cations are expected to bind to certain different anions preferentially. At this time, such varioations of the THP salt are not commercially available and could not be tested.

Though typically used only to brighten recycle paper stock, if desired, the wet pulp may have additional components added to it without noticeably decreasing the effectiveness of the THP salt, and in most instances increasing the opacity of the paper made on a conventional papermachine by from 1 to 5% relative to the opacity of paper made with fibers from the same wood pulp which was untreated with the THP salt.

It will now be evident that the process described herein is most effective with high yield pulp. However, wet pulp which has less than 60% cellulose may also be effectively treated to increase its brightness by at least 0.5 point and without sacrificing opacity.

In addition, it may be desirable to further upgrade the recycle wet pulp by addition of a binder and white pigment, particularly an opacifying white pigment, and may additionally contain further usual assistants such as dispersing agents, iron-ion-binders, antifoaming agents etc.

The binders may be any binders such as commonly used for the production of high quality paper, and may include a single binder (P1), preferably a synthetic latex, or, a mixture of primary (B1) and secondary (B2) binders. Binders are usually polymers and copolymers of ethylenically unsaturated compounds, mainly the following: copolymers of butadiene and styrene, optionally containing additionally a carboxylated comonomer, polyacrylates, copolymers of alkyl acrylate and vinyl acetate and/or acrylic acid, itaconic acid or maleic acid and polyvinyl acetates that contain an acrylate. Secondary binders are typified by starch or casein (which may also be partially oxydized and/or hydrolyzed), modified cellulose, e.g. carboxymethyl cellulose and cellulose methylether, polyvinyl alcohol and low molecular carboxyl group containing polymers (in particular also polycarboxylic acid, e.g. copolymers of acrylic acid C₁₋₄-alkyl ester and acrylic acid which may additionally function as dispersing agents and as iron-ion-binders); of these are mainly preferred casein, the carboxy group containing polymers and polyvinyl alcohol. Together with the synthetic latices, may be used common dispersing agents in addition to the polycarboxy compunds, or in place of them, in order to disperse the latices in water; preferably these common dispersing agents are non-ionic emulsifiers. The synthetic binders are commercially available in the form of dispersions that optionally contain a dispersing agent (e.g. from the polymerisation reaction in aqueous dispersion).

White, substantially opacifying pigments are in general inorganic minerals, mainly calcium carbonate, calcium sulphate (satin white) aluminium silicates and aluminium hydroxide, aluminium magnesium silicate (china clay) titanium dioxide or barium sulphate (blanc fix) and mixtures thereof.

Though, in general, there is no need for using a conventional bleaching agent in conjunction with the THP salt, it is found that the interaction of small amounts of conventional bleaching agents up to about 4% by wt of the wet pulp, do not diminish the effectiveness of the THPS salt, except that with sodium hydrosulfte and sodium bisulfite, more than 1% by wt adversely affects the effectiveness of the THP salt.

Having thus provided a general discussion, described the overall process in detail and illustrated the invention with specific illustrations of the best mode of making and using it, it will be evident that the invention has provided an effective substitution for conventional bleaching agents used in the art. It is therefore to be understood that no undue restrictions are to be imposed by reason of the specific embodiments illustrated and discussed, and particularly that the invention is not restricted to a slavish adherence to the details set forth herein. 

1. A process for enhancing the brightness of an aqueous dispersion of wood pulp comprising contacting wet pulp with a tetrakis(hydroxyalkyl) phosphonium (“THP”) salt in an amount greater than 0.04% by weight of the wet pulp, sufficient to enhance the brightness of paper made from the pulp to a desired brightness level, but no more than 5%, the phosphonium compound being represented by the structural formula:

wherein R represents alkyl having from 1 to 4 carbon atoms; and, X represents a suitable and desirable anion selected from the group consisting of HCO₃ ⁻¹, CO₃ ⁻², PO₄ ⁻³, HPO₄ ⁻¹, HPO₄ ⁻², OH⁻¹, Cl⁻¹, Br⁻¹, HSO₃ ⁻¹, SO₃ ⁻², HSO₄ ⁻¹, SO₄ ⁻², CH₃CO₂ ⁻¹ and CH₃SO₄ ⁻¹.
 2. The process of claim 1 wherein the THP salt is present in an amount in the range from about 1% to 2.5% by weight of the wet pulp.
 3. The process of claim 1 wherein the pH of the wet pulp is in the range from neutral to 12, and treated wet pulp provides fibers having a brightness at least 0.5 point higher than a brightness measured for fibers of the same pulp without being contacted with the THP.
 4. The process of claim 1, including feeding the aqueous dispersion of wood pulp to a papermachine, and forming paper without loss of opacity relative to paper made with the same wood pulp untreated with the THP salt.
 5. The process of claim 3 wherein X⁻ is SO₄ ⁻² and R is selected from the group consisting of methyl (—CH₂ ) and ethyl (—C₂H₄).
 6. The process of claim 1, including feeding the aqueous dispersion of wood pulp as wet pulp to a papermachine, and forming paper having at least the same opacity as that of paper made with the same wood pulp untreated with the THP salt.
 7. The process of claim 3 including heating the wet pulp to a temperature in the range from 30° C. to 90° C., and wherein the pH is in the range 4 to
 9. 8. A process for brightening deinked wood pulp in a wet pulp of recycle paper comprising, (i) treating the wet pulp with at least enough of a tetrakis(hydroxyalkyl) phosphonium (“THP”) salt represented by the formula

wherein R represents alkyl having from 1 to 4 carbon atoms; and, X represents a suitable and desirable anion selected from the group consisting of HCO₃ ⁻¹, CO₃ ⁻², PO₄ ⁻³, HPO₄ ⁻¹, HPO₄ ⁻², OH⁻¹, Cl⁻¹, Br⁻¹, HSO₃ ⁻¹, SO₃ ⁻², HSO₄ ⁻¹, SO₄ ⁻², CH₃CO₂ ⁻¹ and CH₃SO₄ ⁻¹; to result in a concentration of from 0.05% to 5% THP in the wet pulp; (ii) maintaining the pulp in contact with the THP for from about 10 to 90 minutes to provide sufficient time for the fibers in the pulp to sorb the THP; (iii) forming THP-coated fibers; feeding wet pulp containing from about 0.1 to 15% by weight of the THP-coated fibers, to a paper machine, to form a web of fibers of wet wood pulp; (iv) removing water from the wet wood pulp; (v) drying the resulting web of paper; and, (vi) measuring a brightness of the web of paper which brightness is at least 0.5 point higher than a brightness measured for paper of the same pulp without being contacted with the THP.
 9. The process of claim 8 wherein the THP salt is present in an amount in the range from about 1% to 2.5% by weight of the wet pulp.
 10. The process of claim 8 wherein the pH of the wet pulp is in the range from neutral to 12, and treated wet pulp provides fibers having a brightness at least 0.5 point higher than a brightness measured for the same pulp without being contacted with the THP.
 11. The process of claim 8, including feeding the aqueous dispersion of wood pulp to a papermachine, and forming paper without loss of opacity relative to paper made with the same wood pulp untreated with the THP salt.
 12. The process of claim 10 wherein X⁻ is SO₄ ⁻² and R is selected from the group consisting of methyl (—CH₂ ) and ethyl (—C₂H₄).
 13. The process of claim 8, including feeding the aqueous dispersion of wood pulp as wet pulp to a papermachine, and forming paper having at least the same opacity as that of paper made with the same wood pulp untreated with the THP salt.
 14. The process of claim 12 including heating the wet pulp to a temperature in the range from 30° C. to 90° C., and wherein the pH is in the range 4 to
 9. 15. The process of claim 13, wherein the opacity of paper formed from treated fibers of wood pulp is in the range from 1 to 10% higher than the opacity of paper form from fibers of the same wood pulp except that it is untreated with the THP salt. 